Juno spacecraft reveals a more complex Jupiter

Jupiter’s scientific portrait is getting repainted.

NASA’s Juno spacecraft swooped within about 5,000 kilometers of Jupiter’s cloud tops on August 27, 2016, giving scientists their first intimate look at the gas giant. The data are revealing surprising details about Jupiter’s gravity, powerful magnetic field and ammonia-rich weather system. The findings, which appear in two studies in the May 26 Science, suggest researchers may not only need to revamp their view of Jupiter but also their ideas about how planetary systems form and evolve.

“We went in with a preconceived notion of how Jupiter worked, and I would say we have to eat some humble pie,” says Juno mission leader Scott Bolton, a planetary scientist at the Southwest Research Institute in San Antonio.

Scientists thought that beneath its thick clouds, Jupiter would be uniform and boring. But Juno revealed the planet is anything but, Bolton says. “Jupiter is much more complex deep down than anyone anticipated.”

For starters, measurements of Jupiter’s gravity, determined from the tug of the planet on the spacecraft, suggest that the planet doesn’t have a solid, compact core, Bolton and colleagues report in one of the new papers. Instead, the core is probably large and diffuse, possibly as big as half the planet’s radius, the team concludes. “Nobody anticipated that,” Bolton says.

Imke de Pater, a planetary scientist at the University of California, Berkeley who was not involved in the studies, says the new gravity measurements should allow scientists to get a better handle on the structure of the planet’s core. But, she notes, because of the mathematics involved, it won’t be an easy task.

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She was even more surprised by new measurements of Jupiter’s magnetic field, which is the strongest in the solar system. The Juno data reveal the magnetic field is almost twice as strong as expected in some places. But the field’s strength varies from location to location, growing stronger than expected in some areas and weaker in others. The data support the idea that the magnetic field originates from circulating electric currents in one of the planet’s outer layers of molecular hydrogen.

In a complementary paper, astrophysicist John Connerney of NASA’s Goddard Space Flight Center in Greenbelt, Md., and colleagues look at how Jupiter’s magnetic field interacts with the solar wind, a stream of charged particles flowing from the sun. That interaction influences Jupiter’s auroras, which Juno captured in ultraviolet and infrared images. Studying the brilliant light shows at the planet’s poles, the team observed particles falling into the planet’s atmosphere, similar to what happens on Earth. But there were also beams of electrons actually shooting out of Jupiter’s atmosphere, which isn’t seen on Earth. The finding suggests the gas giant interacts very differently with the solar wind, the team writes.

Another oddity, described by Bolton’s team, is how ammonia wells up from the depths of Jupiter’s atmosphere. The upwelling resembles a feature on Earth called a Hadley cell, where warm air at our equator rises and creates trade winds, hurricanes and other forms of weather. Jupiter’s ammonia cycling looks similar. But because Jupiter lacks a solid surface, the upwelling probably works in a completely different way than on Earth. Figuring out how the phenomenon occurs on Jupiter may help scientists better understand the atmospheres of other planets.

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Jupiter is a standard of comparison for all gas giants, within and beyond the solar system. “What we learn about Jupiter will impact our understanding of all giant planets,” Bolton says. Most planetary systems have Jupiter-like planets. By helping researchers determine how the one in our solar system formed and operates, the new data could give clues to how other planetary systems evolved as well.